1. Field of the Invention.
The instant invention relates to producing thick anodic coatings on aluminum. More specifically it relates to an anodizing bath for aluminum substrates which results in substrates suitable for electronic circuit packaging applications.
2. Description of the Prior Art.
Many prior art processes are known for anodizing aluminum and its alloys. U.S. Pat. No. 3,985,629 to Kimura discloses a method for color-anodizing aluminum and its alloys, in which the anodizing takes place in an electrolytic bath including citric and sulfuric acids.
U.S. Pat. No. 3,849,264 to Weber et al discloses conventional anodizing of aluminum utilizing an aqueous electrolyte containing preferably from about 11% to 17% by weight of sulfuric acid.
U.S. Pat. No. 4,229,266 to Usbeck relates to anodically oxidizing aluminum and its alloys in an aqueous electrolyte containing sulfuric and phosphoric acids after a previous mechanical, chemical or electrochemical roughening. Sulfuric acid is present in a concentration ranging from about 25 to 150 grams per liter: phosphoric acid, ranging from about 10 to 50 grams per liter. This reference also discusses pretreatment including grinding, polishing, brushing or blast abrasion, or chemical surface treatment for degreasing, pickling or producing a matte surface or an electrochemical surface treatment by the action of an electric current in an acid. The teachings of this reference specifically are aimed at preparing materials for use as printing plate supports.
U.S. Pat. No. 4,406,757 to Huang relates to a method of electrolytically treating lithographic grade aluminum in a sulfuric acid electrolyte where the temperature is maintained at less than about 50 degrees centigrade.
U.S. Pat. No. 4,439,287 to Birkle et al relates to a method for anodizing aluminum in a bath having a temperature of about 0 to 15 degrees centigrade for producing an anodized layer more than 4 microns thick, which is hard and abrasion proof.
U.S. Pat. No. 4,478,689 to Loch discloses a method an apparatus for electrolytically processing aluminum and its alloys, including means for automatically sensing the process voltage applied to the surface to be anodized and adjusting the duration of anodizing current pulses accordingly.
U.S. Pat. No. 4,481,083 to Ball et al relates to a process for anodizing aluminum for electrolytic capacitors.
U.S. Pat. No. 4,507,179 to Yoshida et al relates to an aluminum substrate suitable for making high-density magnetic recording media produced by anodizing a surface of the aluminum substrate in an aqueous chromic acid solution at an electrolytic voltage higher than 60 volts.
IBM TDB Vol. 23, No. 8, January, 1981, pp. 3854-3856 to Homola et al discloses use of colloidal silica to seal anodized aluminum surfaces.
None of these examples of the prior art relate to preparing by electrolytic anodization, aluminum having particular utility as substrates in the manufacture of printed circuitry. All these patents and available literature teach methods in which the anodic coating has a hybrid structure primarily of the crystalline type. The coating, particularly those with thicknesses above 25 microns, have chemical as well as mechanical flaws which render them unsuitable in circuit packaging.
Finally, while the above references are believed to be pertinent to the present invention, many patents and other prior art relate to anodizing aluminum. None however, yields the results of the present anodizing process which includes a novel anodizing bath.